Upper limit on the $\eta \to {\pi }^{+}{\pi }^{-}$ branching fraction with the KLOE experiment

Based on an integrated luminosity of 1.61 fb${}^{-1}$ $e^{+}{e}^{-}$ collision data collected with the KLOE detector at DA$\mathrm{\Phi }$NE, the Frascati $\varphi$-factory, a search for the $P$- and $CP$-violating decay $\eta \to {\pi }^{+}{\pi }^{-}$ has been performed. Radiative $\varphi \to \eta \gamma$ decay is exploited to access the $\eta$ mesons. No signal is observed in the ${\pi }^{+}{\pi }^{-}$ invariant mass spectrum, and the upper limit on the branching fraction at 90% confidence level is determined to be ${\mathcal B}(\eta\to\pi^{+}\pi^{-})<4.9\times10^{-6}$, which is approximately three times smaller than the previous KLOE result. From the combination of these two measurements we get $\mathcal{B}(\eta \to {\pi }^{+}{\pi }^{-})<4.4\times {10}^{-6}$ at 90% confidence level.